Hypomagnesaemia
NHS Scotland TAM (Treatments and Medicines) NHS Highland Hypomagnesaemia (Guidelines) Last reviewed: Apr 2022.
Background Information
Definition
Hypomagnesaemia is defined as serum magnesium <0.7 mmol/L (normal range: 0.7-1.0 mmol/L)
Aetiology
Key causes: [Ref]
- GI loss (e.g. diarrhoea, vomiting, malabsorption, laxative abuse)
- Inadequate dietary intake
- Chronic alcohol use (due to decreased intake, GI loss, and alcohol-induced renal damage)
- Drugs
- PPIs (e.g. omeprazole)
- Diuretics
- Pre-eclampsia and eclampsia (hypomagnesemia does not cause preeclampsia or eclampsia, they are just associated with altered magnesium homeostasis)
- Congenital renal magnesium wasting (e.g. Bartter syndrome, Gitelman syndrome)
Clinical Manifestations
Clinical features of hypomagnesaemia are non-specific and often overlooked as serum magnesium is not routinely measured in clinical practice. [Ref]
| Neuromuscular manifestation | Mild:
Severe (neuromuscular manifestations):
|
| Cardiac manifestation |
*Electrolyte causes of QTc prolongation are the hypos – hypomagnesaemia, hypokalaemia, hypocalcaemia |
| Metabolic manifestation | Hypomagnesaemia can cause secondary:
|
Investigations:
- ECG (very important in moderate to severe hypomagnesaemia to identify ECG changes)
- Potassium and calcium levels (to identify co-existing hypocalcaemia and hypokalaemia)
Management
Moderate Hypomagnesaemia (0.5-0.7 mmol/L)
Treat with oral magnesium (e.g. magnesium aspartate dihydrate powder sachet)
- To be continued until 48 hours after magnesium levels have normalised
Severe Hypomagnesaemia (<0.5 mmol/L)
Treat with IV magnesium sulfate
- IV magnesium should be given as an infusion
- The only exception is in haemodynamically unstable patients who should have magnesium IV boluses (1-2g over 2-5 15min)
It is important to monitor for clinical signs of magnesium toxicity:
- Serum magnesium levels
- Deep tendon reflex (reduced deep tendon reflex) – the earliest and most sensitive indicator
- Blood pressure and pulse (hypotension and bradycardia)
- Respiratory rate (respiratory depression)
- Urine output (poor urine output can contribute to toxicity)
- ECG monitoring (not routinely indicated in stable patients)
Management of magnesium toxicity: stop the magnesium infusion + IV calcium gluconate (antidote)
Patients with eGFR <30 are at risk of magnesium toxicity (as magnesium is renally excreted), they should receive ~50% of the normal magnesium replacement dose.